Truck hunting due to the conical configuration of the wheel treads causes the rolling wheels and axle assembly of a standard railroad freight car truck to move along a pair of rails in a generally sinusoidal path, causing the railway truck to oscillate laterally and yaw cyclically with respect to the car body about a vertical axis passing through the truck bolster center plate. With controlled truck hunting, the amount of the cyclical lateral motion is relatively small and the wheel flanges will not contact the rails. However, at higher speeds, unstable truck hunting can develop if the frequency of the cyclical motion approaches resonance, wherein the wave pattern of the wheel and axle assembly has the same frequency as the natural roll, sway, and yaw frequencies of the car body. Violent lateral forces are created during uncontrolled truck hunting, which can cause rail damage, excess wear to the truck and car body, as well as heavy lateral impacts between the wheel flanges and rails.
Railway truck side bearings, located laterally outward of the truck center plate, have been utilized in the prior art to provide lateral stability of the car body with respect to a truck. Such side bearings are necessary to limit the amplitude of car body lateral roll motion as can occur under low speed lateral harmonic roll conditions or the degree of tilt of the car body as can occur with the car slowly transversing superelevated track curves.
Typical roller side bearings in the prior art include rollers carried within a housing mounted on the railway truck bolster. The roller extended above the open top of the housing for rolling engagement when contacted by a wear plate on the bottom of the car body. In this way, the car body was stabilized laterally outwardly of the truck center plate on the bolster, while permitting the truck to rotate about a vertical centerline of the bolster center plate, to permit normal truck movement along the railroad track.
In an effort to improve upon the conventional side bearings, so as to increase truck hunting stability as well as car body lateral roll stability, various devices have been installed between the car body and truck bolster to replace or supplement the conventional roller side bearing. Such devices have ordinarily taken the form of elastomeric blocks or other elements forming a constant contact bearing to modify the rotational swiveling resistance characteristics of the truck as well as affecting the lateral roll stability of the car body.
As a result of prior art truck stabilizing devices, the truck swiveling resistance was usually characterized by a gradually increasing opposed force up to a point of relative sliding displacement between the support surfaces. As the truck continued to swivel, a substantially constant sliding frictional restraint was maintained up to the point of maximum truck swiveling. Because of the relative sliding displacement, the return truck motion was opposed by a frictional sliding resistance. Also, because of the reverse elastic deformation of the elastomeric block, some additional truck swiveling motion was required to restart the truck in a straight running position.